Method for forming a stress-free multilayer PVC sheet material

ABSTRACT

A method for forming a multilayer plastic sheet material ( 1 ) for floor and/or wall panels, wherein a first polymer mass comprising a rigid PVC is melted under pressure and is passed through an extruder head at a specified discharge rate in the form of a plastic strand in sheet form that is provided with one or more layers so that a multilayer plastic strand is formed, which is passed to two or more rolls of a finishing stand, which processes the multilayer plastic strand into a sheet of defined thickness, which is then led away via a transport device to a sawing device to be cut to the desired length, wherein, after the plastic strand in sheet form leaves the extruder head, it is first passed between a top roll and a bottom roll of a roughing stand, wherein the speed of the rolls of the finishing stand and the rolls of the roughing stand is synchronized with the discharge rate of the plastic strand in sheet form from the extruder head, so that said plastic strand is processed without stress.

This application is a National Phase entry of International ApplicationNo. PCT/IB2018/052855 under § 371 and claims the benefit of Belgianpatent application No. BE-2017/5285, filed Apr. 24, 2017, which ishereby incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

The present invention relates, on the one hand, to a method for forminga multilayer plastic sheet material for floor and/or wall panels,wherein a first polymer mass comprising a rigid PVC is melted underpressure and is forced through an extruder head at a specified dischargerate in the form of a plastic strand in sheet form that is provided withone or more layers so that a multilayer plastic strand is formed, whichis propelled towards two or more rolls of a finishing stand, whichprocesses the multilayer plastic strand into a sheet of definedthickness, which is then led away via a transport device to a sawingdevice to be cut to the desired length, wherein, after the plasticstrand in sheet form leaves the extruder head, it is first passedbetween a top roll and a bottom roll of a roughing stand, wherein thespeed of the rolls of the finishing stand and the rolls of the roughingstand is synchronized with the discharge rate of the plastic strand insheet form from the extruder head, so that said plastic strand isprocessed without stress. On the other hand, the present inventionrelates to a multilayer sheet material with an elastic modulus (E) above1000 N/mm², preferably between 2000 N/mm² and 3000 N/mm², and aspecified length and width.

The present invention relates in particular to a method that allows amultilayer PVC sheet material to be formed with minimum possibleinternal stresses.

BACKGROUND

The sheet material that is manufactured by said method is in particulara plastic panel manufactured from PVC, which can be both a floor paneland a wall panel. The thickness of such panels is preferably between 2and 10 mm, in particular between 3 and 8 mm. The panels that aremanufactured as standard preferably have a width between 5 and 300 cmand a length between 10 and 500 cm.

The known panels are manufactured by an extrusion process. Adisadvantage of this technique is that stresses are built up in thefinished material. There is consequently a risk of subsequentdevelopment of fine cracks in the material.

The Japanese patent publications JP 2002 234116A and JP H07 100898Adescribe a known extrusion process.

SUMMARY

An aim of embodiments of the present invention is to supply a methodthat will allow stress-free manufacture of sheet material.

This aim may be achieved by a method for forming a multilayer plasticsheet material for floor and/or wall panels, wherein a first polymermass comprising a rigid PVC is melted under pressure and is passedthrough an extruder head at a specified discharge rate in the form of aplastic strand in sheet form that is provided with one or more layers sothat a multilayer plastic strand is formed, which is passed to two ormore rolls of a finishing stand, which processes the multilayer plasticstrand into a sheet of defined thickness, which is then led away via atransport device to a sawing device to be cut to the desired length,wherein, after the plastic strand in sheet form leaves the extruderhead, it is first passed between a top roll and a bottom roll of aroughing stand, wherein the speed of the rolls of the finishing standand the rolls of the roughing stand is synchronized with the dischargerate of the plastic strand in sheet form from the extruder head, so thatsaid plastic strand is processed without stress, and wherein a secondpolymer mass comprising a flexible PVC is melted under pressure and isco-extruded on the first polymer mass, and then both are passed throughthe extruder head in the form of a multilayer plastic strand in sheetform, the top layer of which is formed by the second polymer mass.

In a particular embodiment of the method according to the invention, thesecond polymer mass comprising a flexible PVC is melted under pressureand is co-extruded on the underside of the first polymer mass, and bothare then passed through the extruder head in the form of a multilayerplastic strand in sheet form, this additional second polymer massforming a stabilizing layer for the multilayer sheet material. Thestabilizing layer preferably has the same thickness as the top layer.The stabilizing layer will prevent warping of the sheet material.

Preferred embodiments of the method are described in the dependentclaims.

The present invention in some embodiments further relates to amultilayer sheet material with an elastic modulus (E) above 1000 N/mm²,preferably between 2000 N/mm² and 3000 N/mm² and a specified length andwidth, wherein said sheet material comprises a carrier material formedfrom rigid PVC provided with one or more layers, wherein the sheetmaterial, after heating to 80° and cooling back to room temperature, hasexpansion or shrinkage of max. 0.2% on the initial length and/or widthand wherein the carrier material formed from rigid PVC is provided witha top layer of flexible PVC. Preferably, after heating to 80° andcooling back to room temperature, the sheet material has expansion orshrinkage of max. 0.1% on the initial length and/or width.

Preferred embodiments of the sheet material according to the inventionare described in the dependent claims.

To explain the properties of this invention and to indicate additionaladvantages and particular features thereof, a more detailed descriptionof the method and the sheet material according to the invention ispresented below. It should be pointed out that nothing in the followingdescription is to be interpreted as a limitation of the protectionclaimed in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

This description is explained with the aid of the appended drawings, inwhich:

FIG. 1 : a schematic representation of a multilayer sheet materialaccording to the invention;

FIG. 2 : shows schematically a part of the device in which the methodaccording to the invention is applied;

FIG. 3 : explains the application of a structure in the wearing layer;

FIG. 4 : shows the transport device along which the formed multilayersheet material is led away to a sawing device;

FIG. 5 : shows schematically the application of a bottom layer againstthe underside of the sheet material.

DETAILED DESCRIPTION

A sheet material (1) according to some embodiments of the presentinvention is a PVC sheet built up from several layers, which willpreferably be used as a waterproof and low-noise floor or wall covering.

As can be seen from FIG. 1 , the sheet (1) is built up as follows:

-   -   1) UV varnish coat (6), about 25 g/m²;    -   2) wearing layer (5);    -   3) decorative layer (4) (film) made of PVC (may optionally also        be PP or PE);    -   4) flexible PVC layer (3);    -   5) rigid PVC layer (2), which functions as a carrier or base        sheet;    -   6) layer of adhesive (7), preferably an EVA adhesive (other        types of adhesive may also be considered);    -   7) bottom layer (8), preferably made of a foamed material.

Obviously sheet materials with some other structure likewise fall withinthe scope of protection of this invention, such as for example a sheetmaterial where a decorative layer (4) is not present and the decorativeeffect is provided by providing the wearing layer (5) (preferably on theunderside) with printing or where the wearing layer is omitted for thewall application and the decorative effect is applied via the decorativelayer (4) or by direct printing on the flexible PVC layer (3) or on therigid PVC layer (2) before applying the UV varnish coat (6).

The rigid PVC layer (2) of the sheet material may also be provided onits underside with a flexible PVC layer, which functions as astabilization (stabilizing) layer. The stabilizing layer preferably hasthe same thickness as the flexible PVC layer that forms the top layer(3). The stabilizing layer is applied at the same time as the top layer.Through the presence of the stabilizing layer, the sheet material willbe less susceptible to the so-called “bimetal effect” and the sheet willremain straight and will not warp.

The various layers from which the sheet is built up may be manufactured(applied) in a single production process, and if a structure is alsoapplied in the surface, this may for example be a wooden or stonestructure. The method in an embodiment is described below, along withthe precise points requiring attention in order to keep it stress-free.

Step 1: Manufacture of the Carrier Sheet:

The basis or the core of the sheet (1) is the layer of rigid PVC (2)provided with a layer (3) of flexible PVC. Both layers are manufacturedby means of a so-called multilayer T-die (9). Said die is connected totwo separate extruders. The main extruder supplies the material for therigid PVC layer (2), and a second extruder, the so-called co-extruder,supplies the material for the flexible PVC layer (3). The two layers aredistributed in the die over a sheet width with a specified thickness anddensity.

A rigid PVC consisting of various components is processed in the mainextruder. Fillers and minerals may also be added via a mixer on top ofthe extruder, in order to obtain optimum thermal stability of the sheet(1). Addition of a chemical blowing agent provides small air bubbles inthe sheet, thus reducing the specific weight of the sheet. During theextrusion process, the additives are mixed and melted with the PVC, thismelt enters the die at a specified pressure and temperature and isspread by the “hat-rack” principle over the full width of the die andwith a specified thickness.

The action of the co-extruder is similar; the material fed into theco-extruder is a flexible PVC with a specified Shore hardness. The Shorehardness of the flexible PVC is preferably between 80 and 100, moreparticularly between 85 and 95. The purpose of the flexible PVC layer ison the one hand to guarantee perfect thermal fusion with the decorativefilm (see hereunder) and on the other hand to create a soft feel.

It is important that the flows of the two layers in the die advance atthe same speed, to avoid stresses and warping of the sheet duringsubsequent thermal loading. The lip opening of the die (where thematerial is forced out) must be set according to a specified ratio sothat the final thickness of the co-extruded sheet leaves the die withina specified thickness ratio. If these speeds are too different over thewidth of the die, the shrinkage ratios on cooling of the sheet will beinsufficient to be able to guarantee a good, stable floor.

Step 2, Pre-Thickness Calibration of Sheet+Pre-Lamination of Decorative(Film) Layer and Wearing Layer (see FIG. 2 )

Once the melt comes out of the die (9), this melt is received by tworolls (10, 11) that form part of a roughing stand, namely a top roll(10) and a bottom roll (11). A specific feature of these rolls is thatthey must be maintained at a specified temperature so that the hot meltleaving the die (9) is not cooled immediately owing to an excessivetemperature difference between the rolls and the hot melt. A specificcomposition of silicone rolls is selected for this purpose.

The operation of the roughing stand is as follows:

-   -   a) a first pre-calibration in thickness of the melt leaving the        T-die (9), together with the applied decorative layer (4) and        wearing layer (5). The gap between the two rolls (10, 11) must        therefore be very accurate so that the rolls (10, 11) do not        press excessively on the sheet, which is still hot.    -   b) light pressing of the decorative layer on the hot sheet. The        decorative layer is preferably a layer of PVC film. This PVC        film comes from a roll produced beforehand. The unwinding of the        film takes place over a number of rolls, the roll RPVC 1 is a        roll that measures the stress constantly on the decorative layer        (PVC film) (4) and ensures that the forces exerted that are        necessary for unwinding the PVC film are always the same,        regardless of the diameter of the roll of PVC film or the film        thickness. The roll RPVC 2 is located at a specified angle. The        purpose of this roll is to bring the film under the wearing        layer (5) and between the rolls (10 and 11) of the roughing        stand, in such a way that there is no inclusion of air between        the PVC film (4) on the one hand and the wearing layer (5) and        the extruded PVC sheet. Temperature, stresses, angle and        distance are very important for obtaining a stable process. Of        course, the whole unwinding unit is provided with continuous web        tensioning and web guidance.    -   c) Applying the wearing layer (5) on the 3 layers already        present: the decorative film, flexible PVC layer and rigid PVC        layer. The wearing layer (5) is also unwound by a complete        unwinding unit provided with web tensioning and web guidance.        However, a number of points have to be taken into account.        -   1) Depending on the thickness of the wearing layer (5), it            must be warmed up to some extent for example by means of IR            radiant heaters (12), to make the whole a bit more flexible            but also to limit the thermal shock when they are laminated            together; the melt emerging from the die (9) has a            temperature of about 150°, the decorative film (4) is so            thin that it reaches this temperature immediately, but the            wearing layer (5) has thicknesses from 0.1 to 1.0 mm and            must therefore be warmed up somewhat. This is provided by            the IRWL radiant heaters (12).        -   2) After this preheating, the film passes over a roll stand            RWL2 and RWL3, and once again angles, distances and stresses            are extremely important. The aim is that the preheated            wearing layer is entrained almost stress-free by the top            roll (10), and an important aspect is the contact surface or            the angle at which the wearing layer comes into contact with            the top roll.    -   d) Finally, the top rolls (10 and 11) will press the melt        emerging from the die (9), the PVC decorative film and the        wearing layer together. The gap between the rolls (10, 11) is        important. It must be in proportion to the thickness of the melt        emerging from the die and the thickness of the film and the        wearing layer that are to be pressed together. If there is        insufficient contact, air will be trapped between the different        layers. With excessive contact or excessive squeezing together,        creases may form on compressed sheet that are visible to the        naked eye, but mechanical forces will also develop in the hot        state, which will be harmful later, possibly with negative        thermal stability as the sheet cools down.    -   e) It is therefore very important that the decorative film and        the wearing layer are unwound with a certain tension, just        sufficient for everything to be fused together without inclusion        of air and without stretching the film and the wearing layer. On        the other hand, it is also important that the melt emerging from        the die is brought stress-free between the top roll and the        bottom roll. Moreover, it is extremely important that the top        roll and bottom roll speeds can be set accurately and        separately. The rolls C1, C2 and C3 of the finishing stand,        which function as master, are decisive for the speeds of the top        and bottom rolls of the roughing stand. The latter are now        synchronized with the speed of rolls C1, C2 and C3 with the        possibility of setting a certain delta between the two speeds to        prevent the development of stresses. In its turn, the extrusion        speed, with the associated discharge rate of the melt from the        T-die, is also synchronized with the top and bottom rolls of the        roughing stand also taking into account a possible delta on        these speeds, once again to prevent the possible development of        stresses.

Step 3: Final Calibration of the Thickness and Introduction of aStructured Surface (Embossing) (See FIG. 3 )

Once the aforementioned layers have been (pre)laminated on one another,they still need to be fused together, better calibrated on thickness andthe surface must be provided with a defined embossing structure. Thismust all be done at specified temperatures and of course with minimumstresses.

After the roughing stand, the sheet (1) passes over 2 driven supportrolls (S1 and S2). This interval is needed for carrying out thefollowing two operations:

-   -   1) Completion of thermal fusion of all the layers. It is of        course important that the various layers used are compatible        (fusible) with each other with respect to chemical composition.    -   2) Softening (warming) the top layer of the sheet for final        impressing of an attractive surface structure (embossing) with        roll C2. Thus, the top part of the wearing layer (5) is made        soft again by means of a second IR radiant heater (13). The        lamps of this radiant heater (13) have a specific wavelength        that only softens the top part of the wearing layer. It is very        important not to heat up the whole wearing layer again. That is        why IR lamps with a specific wavelength are used for this. The        temperature of the surface after the IR radiant heaters is        measured continuously and is kept constant by means of a PID        control system. It is very important at this point to have a        constant surface temperature over the entire surface of the        sheet, for finally producing an embossing structure that is as        uniform as possible in the sheet by means of roll C2.

Thus, S1 and S2 are only support rolls, there is no top roll that wouldcompress the sheet thickness.

Moreover, it is extremely important that after it leaves the die andduring its subsequent path the sheet is not under tension. In otherwords this sheet must not be pulled, otherwise excessive stresses willbe created in the sheet. On the other hand it is necessary to ensurethat the sheet also does not sag excessively. In that case the sheetwill be stretched under its own weight and it is impossible to bringabout a stable process. Very good synchronization with the necessaryoffset possibilities between on the one hand the speed of discharge fromthe T-die and the speed of all the aforementioned rolls is thus of theutmost importance on the one hand for obtaining a stable thickness andon the other hand to produce minimum mechanical stresses in the sheet.

After the support rolls and heating of the wearing layer with the IRradiant heaters (13), the sheet goes into the rolls of the finishingstand (calender). All three rolls have a specific constant temperature.The middle roll C2 is a structure roll, which will apply a specifiedstructure (embossing) on the surface of the sheet (1). The accuracy orthe sharpness of this structure depends on the thickness of theassembled sheet that is passed between the rolls, but also on thetemperature. Once again it is necessary to find the right conditionsbetween the temperature, pressure and thickness setting between therolls. The slightest deviation of these factors results in a poorerfinish.

The depth of the structure in the structure rolls is normally somewhatdeeper than the depth of the structure in the final sheet. The greatdifficulty in this method of production is that a structure is impressedin the sheet while it is still almost completely soft (has nothardened). The sheet can thus be flattened without much difficulty,which in itself would make the process a bit easier. However, experienceand tests teach us that this then introduces a very large mechanicalload into the sheet, if we form the sheet and cool it with theseintroduced stresses, then the thermal properties that we obtain aftercooling are actually very poor.

Step 4, Stress-Free Cooling of the Sheet on a Long Roller Table at aSpecified Angle (see FIG. 4 )

This process has already been described in an earlier patent publicationof the patent holder. Here, the sheet will be cooled on a long rollertable that is set up at an angle. At the end of this roller table thereare two driven pinch rolls (TR), which ensure that the sheet willadvance at a specified linear speed. The sheet will sag between roll C3and the first roll (15) of the roller table. The value or factor of thesag is measured by measuring sensors. The values of these measurementsare converted and control the peripheral speed of the pinch rolls (TR)via a PID control system. In this way, the value of the sag of the sheet(factor X) will always be kept constant and we are able to state thatthe sheet is cooled stress-free. Once the sheet comes up to the level ofthe pinch rolls it has been cooled to the extent that it is in the solidstate and so is no longer susceptible to certain mechanical stresses towhich it may be subjected in the further course of the process (sawingoff of edge strips, gluing a sound-absorbing bottom layer, sawing tolength).

If we now examine FIGS. 2, 3 and 4 together, we can state that the speedof rolls C1, C2 and C3 is decisive (master) in this process. Thesetherefore have the speed that is selected by the operator. The rollsbefore and after these rolls operate automatically as follower (slave),these therefore adapt, via synchronization and measurement and offsetsystems, their peripheral speed so as to be able to guarantee a completeproduction process with a constant tension. Of course, the dischargerate of the melt from the die (9) must also be constant for this processto be kept stable overall.

Step 5, Gluing of the Bottom Layer (See FIG. 5 )

The bottom layer (8) is preferably a pre-extruded PS foam layer, whichis glued onto the underside of the sheet (1). The purpose of this bottomlayer is generally known in applications of floating floor covering.

The bottom layer comes from a roll, via constant web guidance andconstant web tensioning measured by roll U1, the layer (8) will be gluedto the bottom of our sheet. The layer of adhesive (7) is applied bymeans of a special gluing head (slip nozzle). The bottom layer (8) slipswith a specified force over the slip nozzle, which will apply an amountof glue on the bottom layer; the glue is heated in a reservoir andforced by gear pumps into the slip nozzle over a roll system U2 and U3ensuring a constant area of contact between the outlet of the slipnozzle (14) and the bottom layer (8), so that a constant amount of gluewill be applied on the bottom layer. The bottom layer then goes underthe pinch rolls TR and is thus pressed against the sheet, which isproduced as described above.

The speed of the gear pumps is controlled by the speed of the pinchrolls TR, thus guaranteeing a constant yield/m² glue. Factors such asresidual temperature and open times of the glue must be taken intoaccount.

Constant web guidance ensures that the bottom layer is or will always beglued on the sheet at one and the same tension.

Practically all steps of the production process have now been completed.After gluing, the sheet is trimmed to width, the left and right edgestrips are removed by means of circular saws, cutting or cutting-offdevices so that we finally obtain a constant width of the sheet. Afterremoving the edge strips, the sheet will be reduced to the desiredlength, preferably by means of a sawing, cutting or cutting-off device.

Once the sheet has been reduced to the desired length, it goes through abrush system to remove all residues from the sawing operation, and in alast step the sheets are provided with a UV varnish coat (6). Thisvarnish coat on the one hand provides a matt finish, and on the otherhand this varnish coat is an extra protection against soiling of thefloor.

The varnish coat is applied using classical rotary applicators, twolayers are applied wet in wet, and after applying the varnish these arecured by means of UV lamps.

The invention claimed is:
 1. A method for forming a multilayer plasticsheet material for floor and/or wall panels, comprising: melting, underpressure, a first polymer mass comprising a rigid PVC and a secondpolymer mass comprising a flexible PVC and passing the first and secondpolymer masses through an extruder head at a specified discharge rate inthe form of a multilayer plastic strand in sheet form, the top layer ofwhich is formed by the second polymer mass, passing the multilayerplastic strand to two or more rolls of a calendering device andprocessing the multilayer plastic strand into a sheet of definedthickness, leading away the sheet of defined thickness via a transportdevice to a sawing device to be cut to the desired length, after theplastic strand in sheet form leaves the extruder head, passing theplastic strand in sheet form between a top roll and a bottom roll of apre calendering unit, wherein the speed of the rolls of the calenderingdevice and the rolls of the pre calendering unit is synchronized withthe discharge rate of the plastic strand in sheet form from the extruderhead, further comprising providing the plastic material with adecorative effect by a) applying a decorative layer of PVC on themultilayer plastic strand before the multilayer plastic strand in sheetform is passed between the top roll and the bottom roll of the precalendaring unit, or b) by applying a transparent wear layer with athickness of between 0.1 and 1 mm, which is provided with a print, onthe multilayer plastic strand before the multilayer plastic strand insheet form is passed between the top roll and the bottom roll of the precalendaring unit.
 2. The method according to claim 1, characterized inthat a wear layer with a thickness of between 0.1 and 1 mm is applied onthe decorative layer before the plastic strand in sheet form is passedbetween the top roll and the bottom roll.
 3. The method according toclaim 1, further comprising heating the wear layer before it is applied.4. The method according to claim 1, further comprising deflecting theplastic strand in sheet form between the extruder head and the precalendering unit.
 5. The method according to claim 2, furthercomprising, after the pre calendering unit, passing the plastic strandin sheet form over two support rolls in the direction of the calenderingdevice, partially heating the wear layer of the plastic strand in sheetform again between the support rolls, and then applying a structure inthe wear layer by means of a roll of the calendering device.
 6. Themethod according to claim 1, characterized in that the sheet materialhas an elastic modulus above 1000 N/mm².
 7. The method according toclaim 1, wherein the defined thickness is between 3 and 10 millimeters.8. A method for forming a multilayer plastic sheet material for floorand/or wall panels, comprising: melting, under pressure, a first polymermass comprising a rigid PVC and a second polymer mass comprising aflexible PVC, passing the first polymer mass through a first extruderand passing the second polymer mass through a second extruder, both thefirst and the second extruders connected to a multilayer T-die, whereinthe passing of both the first and the second polymer masses is at aspecified discharge rate in the form of a multilayer plastic strand insheet form, the top layer of which is formed by the second polymer mass,passing the multilayer plastic strand to two or more rolls of acalendering device and processing the multilayer plastic strand into asheet of defined thickness, leading away the sheet of defined thicknessvia a transport device to a sawing device to be cut to the desiredlength, after the plastic strand in sheet form leaves the extruder head,passing the plastic strand in sheet form between a top roll and a bottomroll of a pre calendering unit, wherein the speed of the rolls of thecalendering device and the rolls of the pre calendering unit issynchronized with the discharge rate of the plastic strand in sheet formfrom the extruder head, further comprising providing the plasticmaterial with a decorative effect by a) applying a decorative layer ofPVC on the multilayer plastic strand before the multilayer plasticstrand in sheet form is passed between the top roll and the bottom rollof the pre calendaring unit, or b) by applying a transparent wear layerwith a thickness of between 0.1 and 1 mm, which is provided with aprint, on the multilayer plastic strand before the multilayer plasticstrand in sheet form is passed between the top roll and the bottom rollof the pre calendaring unit.